Thermal compounds explained by the master…

Confused about thermal compounds? There are many variants, but we only use Artic’s compounds, with Arctic Silver 5 being our preferred choice. However, AS5’s little brother, Ceramique, is a cheaper alternative with similar performance. The difference in performance is explained in this Overclockers post by an Arctic guru:

If we were to send 100 of you AS5 and Ceramique, I would expect that 65 to 75 would get the lowest temps with AS5 and 25 to 35 would get the lowest temps with Ceramique.

The contact areas of every CPU and heatsink are different. Some are flat, some are concave to one extent or another, and some are convex to one extent or another. Some surfaces are very smooth, others are slightly irregular and some are rough. The pressure applied to the thermal interface joint varies from heatsink to heatsink both in total and in evenness across the surface.

All these variables affect how well a certain thermal compound works on a given CPU and heatsink. There is much more to the real-world performance of a thermal interface than the bulk thermal conductivity of the compound. How well the compound wets or comes into intimate contact with the mating surfaces is also very important. If Compound A has twice the thermal conductivity of Compound B, but Compound B wets 90% of the mating surfaces while Compound A only wets 40%, Compound B with its lower thermal conductivity, but better wetting, will perform better if the bond line thickness is the same.

And that is the third compound factor affecting real-world performance; bond line thickness. How well is the compound able to squeeze out of the gap and allow as much physical contact as possible between the CPU and heatsink. If a compound spaces the heatsink away from the CPU, the performance could be worse than a compound with lower conductivity and less wetting that forms a much thinner bond line.

With regard to AS5 and Ceramique, AS5 has much higher thermal conductivity, but Ceramique is a bit better at surface wetting and forming a minimum bond line. Which combination of compound factors is most critical in your system depends on the characteristics of the interface gap noted above. Are the mating surfaces smooth or rough? Are they flat, concave or convex? How much pressure is there on the interface and is the pressure even?

If one of you were to bring your system to our lab, we could develop a compound that would work better on it than any of our retail compounds. But since that compound would be tweaked for the specific characteristics of your CPU to heatsink interface, it will probably perform much worse for some other systems where the thermal joint has different characteristics.

Our retail products have to perform well on the widest range of systems. On a scale of 1 to 100, it is much better to have a product perform in the 70 to 85 range on almost every system than that to have one that performs at the 95 range on some systems, but in the 35 range on others.

This is why you will continue to see posts where Ceramique performs better than AS5. On some systems it will. But on more systems, the AS5 will perform better. If Ceramique performs better than AS5 on your system then keep using Ceramique. Just be aware that the rankings may well change should you use a different heatsink or different CPU.

Also keep in mind that the break-in on AS5 is much longer than with Ceramique and the temperature drop can be more dramatic. I have seen systems where Ceramique was better after a week, but by six weeks, AS5 passed it and gave lower temperatures. Most of thermal compound break-in can be traced back to surface wetting and bond line thickness, the two factors where Ceramique has some advantage over AS5 right out of the gate. Since the silver particles in AS5 are much heavier than the fillers in Ceramique, it takes more time and more thermal cycling for AS5 to fully flaunt its superior thermal conductivity.

After saying all of this, I want to mention that Ceramique should never beat AS5 by the 6C noted in the first post of this thread. Since both the idle temperature and full-load temperature both dropped by at least 5C, I suspect that this new board is measuring temperatures about 4C to 5C lower than the old board. Since the rise in temperature across a thermal joint is proportional to the power, the difference between the compounds would be less at idle than it is at full load if all you were seeing was the compound change. If JudgeDredd had changed compounds without changing the motherboard, he probably would have seen no change at idle and 1C at load or possibly 1C at idle and 2C at load.